This spread sheet show how much an iPhone 5 costs over the period of the contract and what the contract offers. You would probably be looking for is to get the cheapest one over the life of the contract. The cheapest one I found was with T mobile and that was for £917.00 over an 18 month period. The one that offers unlimated tariffs and is the cheapest is the T mobile contract that is priced at £1203.00 over the period of the contract. I managed to find all prices on the providers website.                                                                                                                                                 

The websites I used:
http://www.o2.co.uk/iphone/iphone-5
http://www.t-mobile.co.uk/shop/iphone/?ref=iphone5_hero
http://shop.orange.co.uk/mobile-phones/iphone
http://www.vodafone.co.uk/personal/index.htm
http://www.three.co.uk/